Inkjet recording systems enable simple process as compared to other recording systems, and thus have such advantages that full-color images can be easily printed, and even with use of an apparatus having a simple structure, high-resolution images can be obtained.
As inkjet inks, dye-based inks are used in which various types of water-soluble dyes are dissolved in a mixture liquid containing water or water and an organic solvent. Such dye-based inks have a drawback of being poor in light resistance, although they are excellent in color tone sharpness.
In the meanwhile, pigment-based inks in which carbon black and various types of organic pigments are dispersed are more excellent in light resistance than dye-based inks, and thus studies on pigment-based inks are being intensively made.
However, pigment-based inks tend to cause nozzle clogging as compared to dye-based inks.
Pigment inks are usually prepared as follows. A coloring material and a dispersant are preliminarily dispersed in an aqueous solvent to prepare a dispersion, and then, the dispersion is dispersed to a predetermined degree by a medium dispersion device, followed by diluting at a predetermined concentration.
Pigment-water-based aqueous inks usually contain a surfactant and an aqueous resin to disperse a hydrophobic pigment therein, but the reliability of images obtained is extremely low. Then, in view to improving the quality of images, a film-formable resin particle is added into an ink liquid, however, it is difficult to disperse a plurality of components in a state of being finely and stably dispersed for a long period of time. When a dispersant such as a surfactant is used in a large amount in order to stably disperse such a fine particle, a problem inconveniently occurs, for example, air bubbles occur inside an ink tank and or an inkjet head, causing a degradation in the image quality. Studies are made to introduce a hydrophilic group into a surface of a pigment or to use a resin containing a hydrophilic group, but when a plurality of these resins are mixed, the dispersed state is unstable to cause a degradation of storage stability, although these resins are stable in single use thereof.
Many techniques have been proposed to solve the above-mentioned problems.
There are the following techniques, for example, an inkjet recording image excellent in visibility is obtained by using a recording medium which contains a specific fine particle and is provided with an ink receiving part in which the gloss values of white parts and printed parts are specified (e.g., see PTL 1); an ink recorded matter having an image excellent in glossiness, smoothness and texture is obtained by inkjet recording on a recording medium having a surface roughness (Ra) value A using each color ink having a surface roughness (Ra) value B (for example, see PTL 2), a recorded matter having a high image density and less reduction in glossiness is obtained with maintaining excellent dispersion stability but without causing nozzle clogging by printing using an ink, which contains a resin emulsion in an aqueous medium, on a recording paper having an ink receiving layer containing a silica fine particle and/or an alumina fine particle and having a specified glossiness (for example, see PTLs 3 and 4); an ink recorded matter excellent in glossiness is obtained by printing an aqueous color ink on a recording medium whose surface is treated with a coating liquid containing an alumina particle and the like (for example, see PTL 5); and an image quality close to a photograph, in which a reduction in glossiness at an image part can be prevented, is obtained by print-recording on a recording medium having a porous layer as an uppermost surface layer using an ink containing a polymer fine particle containing a water-insoluble or sparsely water soluble coloring material (for example, see PTL 6), and the like.
Besides the above, the following techniques are also reported: a recorded matter having an improved glossiness is obtained by using an inkjet recording ink containing an aqueous dispersion liquid of a microcapsulated pigment in which a pigment particle is covered with a polymer (for example, see PTL 7); an image having a uniform solid-part image density and excellent quality is obtained by printing an aqueous ink having a surface tension of 25 mN/m to 45 mN/m on a recording medium having a specific contact angle (for example, see PTL 8); for the purpose of obtaining a high image density, a water-insoluble coloring material and a chargeable pseudo-resin fine particle which is smaller than the coloring material are incorporated into an ink (for example, see PTL 9); a self-dispersible pigment having a specific oil absorption (DBP) is incorporated into an ink (for example, see PTL 10); and an aqueous carbon black dispersion liquid is used which is composed of a surface-modified carbon black, has a HLB value of from 7 to 18 and contains a nonionic surfactant having an acetylene skeleton (for example, see PTL 11), and the like.
Further, for the purpose of obtaining a high image density and high image quality, there have been reported inventions such as an inkjet ink in which the amount of a surfactant having a HLB equal to or more than 3 and less than 10 is specified (for example, see PTL 12); an ink set composed of a recording liquid containing a coloring material and of a treatment liquid having a specific isoelectric point (for example, see PTL 13), and the like.
Further, in order to stabilize the dispersibility of ink, the following methods have been reported: a method of dispersing a water-dispersible resin having a carboxyl group and a nonionic hydrophilic group in water (for example, see PTL 14); a method of making the polarity of a water-soluble polymer the same as that of a surfactant used or adding a nonionic surfactant to the water-soluble polymer (for example, see PTL 15); a method of making the polarity of a colored ionic polyester resin the same as that of a hydrophilic group of a colorant in an aqueous recording liquid (for example, see PTL 16); a method of making the dispersion polarity of a pigment the same as that of a resin fine particle (for example, see PTL 17), and the like.
Similarly to the above, for the purpose of stabilizing the dispersibility of ink, there have also been proposed an aqueous inkjet ink composition in which an aldehyde naphthalene sulfonate dispersant, a pigment dispersion liquid having such a particle size distribution that at least 70% of pigment particles having a diameter smaller than 0.1 μm and other pigment particles having a diameter equal to or smaller than 0.1 μm, and/or at least one sulfone solvent are contained (for example, see PTL 18); a recording liquid composed of an aqueous medium containing a pigment, a polymer dispersant and a nonionic surfactant (for example, see PTL 19); and using an AB or BAB block copolymer as a dispersant for pigment (for example, see PTLs 20 and 21). Furthermore, it has been proposed to use a specific pigment, a water-soluble resin and a solvent (for example, see PTL 22).
In the meanwhile, as a method of dispersing a pigment, there have been proposed a method of introducing a substituent containing a water-soluble group into a carbon black (for example, see PTL 23); a method in which a water-soluble monomer or the like is polymerized onto the surface of a carbon black (for example, see PTL 24); and a method of subjecting a carbon black to an oxidization treatment (for example, see PTL 25) as well.
Also, a method of ensuring the water resistance and ejection stability by using an ink containing a ternary polymer composed of an oxidized carbon black, an acrylic acid, styrene, and α-methylstyrene is disclosed in PTL 26.
In addition, an inkjet recording liquid is disclosed in PTL 27, which is characterized in that the volume average particle diameter of dispersed particles therein is 30 nm to 200 nm.
However, with respect to the above-mentioned conventional ink liquids, black pigment inks are not sufficient in high image density and do not satisfy the demands, although color pigment inks can meet the demands for high image density.
PTLs 28 to 30 each disclose an example of ink liquid using a bead having a bead diameter of from about 0.05 mm to about 1.0 mm for use in bead mill dispersion treatment, and PTL 31 proposed an ink liquid using an anionic surfactant as a dispersant, and discussed that the molecular weight of the anionic surfactant is preferably in the range of 1,000≦m≦30,000. However, both the proposed ink liquids are insufficient in terms of dispersion stability, and pigment components weak to strong impact during dispersion are poor in stability after being dispersed, and these ink liquids still have drawbacks with regard to ejection stability and the like.
Meanwhile, in order to provide an ink having improved ejection stability, there have been proposed a method in which the water evaporation rate, the viscosity increasing rate and the particle diameter are defined so as to improve the ejection stability of the ink (for example, see PTL 32); an inkjet liquid composition free from clogging at the nozzle tip, in which values of the conductivity, the viscosity and the surface tension of a water-soluble solvent are defined (for example, see PTL 33); and an ink liquid in which a colored fine particle having a quaternalized carboxyl group on its surface and a diameter of 0.5 μm or smaller is contained (for example, see PTL 34).
In the above mentioned Patent Literatures, an inkjet ink, which has a good balance in glossiness between an image part having a high image density and capable of imparting excellent visibility as well as good texture and a non-image part and which is excellent in high image density as well as ejection stability, and a recording medium having the above features have not yet been provided, and further improvements are required therefor. Note that inventions of inkjet inks (PTLs 35 to 37) made by the present inventors have been also known.